A friction clutch assembly or “clutch” of a car or other automobile having a manual transmission is generally located between the engine and the drive train. The assembly normally includes three adjacent annular plates, including a flywheel that is rotatably driven by the crank shaft, a clutch plate (otherwise known as a driven plate), and a pressure plate that is biased by energy storing devices, such as one or more springs, towards the clutch plate and flywheel to clamp the clutch plate between the flywheel and the pressure plate.
The frictional engagement of the coupling faces of the clutch plate with the adjacent rotating coupling faces of the flywheel and the pressure plate allow the clutch plate to transfer power generated by the engine to the remainder of the drive train. However, unless there is some form of dampening in the drive line to dissipate the irregular impulses of the internal combustion petrol or diesel engine, these impulses will create unwanted driveline noise, which occurs due to blacklash between meshed gears in the gearbox. While all undampened engine vibrations will create noise in the gearbox, the driveline noise is particularly evident when the vehicle is in neutral gear and the clutch is engaged. That noise is known in the industry as “gear rollover noise”.
To prevent transmission of engine impulses through to the gearbox whilst the vehicle is in neutral and the clutch is engaged, a clutch plate with dampening, usually in the form of coil springs or other dampening means, can be used. While a clutch assembly would ordinarily already employ dampening springs, known as drive springs, for the transmission of power from the engine to the gearbox, the dampening required for that purpose is different to the dampening required to address gear rollover noise. Accordingly, a separate dampening system is usually required. That separate dampening system must be capable of wide angular displacement at low torque, for example 0.1 to 0.6 Nm/degree or <1 Nm/degree, to dampen gear rollover noise. This differs from the drive springs which require higher torque capacity, ie usually 120% of the maximum vehicle torque capacity and typically 20 to 80 Nm/degree or >20 Nm/degree.
A typical clutch plate includes a splined hub that accepts a splined shaft to transmit engine rotation to the gearbox or transmission. The splined hub can be connected to a flange via an arrangement which provides for limited angular displacement between the hub and the hub flange. Spring dampening between the hub and the hub shaft can be used to dampen gear rollover noise.
In an arrangement of the above kind, the hub flange can be sandwiched between a main plate and a side plate which are fixed together, and whereby the hub flange is driven to rotate when the main plate is shifted into engagement with the flywheel of an engine through a friction material fixed to the circumferential edge of the main plate, The main and side plate assembly (hereinafter the “plate assembly”) and the hub flange are connected by drive springs, to provide limited angular displacement between them. The angular displacement in this case is provided to dampen torsional vibration in the drive mode of the vehicle rather than gear rollover noise when the transmission is in neutral and the clutch is engaged.
Where the dampening system for dampening gear rollover noise is provided between the hub and the hub flange, the dampening has been provided in some prior art arrangements by circumferential compression springs. However, these systems are limited by the small degree of angular displacement they allow between the hub and the hub flange. Typically, the angular displacement which is required to eliminate gear rollover noise is very wide, but the space available to accommodate the circumferential compression springs of the prior art systems is not necessarily sufficient to permit the angular displacement required for complete or substantial dampening of the gear rollover noise. Maximum angular displacement can be achieved by positioning the compression spring at the maximum distance radially away from the centre of the hub. However, the further the spring is positioned away from the hub, the longer the spring is required to be for the same angular displacement. This can create space problems because the distance the springs can be positioned away from the hub is limited by other components of the clutch, such as the drive springs between the hub flange and the plate assembly and the inside diameter of the friction material. Accordingly, there is normally a compromise between obtaining the maximum angular displacement and the length of the spring that can be used.
U.S. Pat. No. 6,029,793 discloses a clutch assembly that includes a dampening arrangement comprising a plurality of coil springs each disposed circumferentially. When a torsional vibration occurs, the various springs compress through relative movement between the input rotary members driven by the flywheel and the output rotary member which drives the transmission shaft. Four different sets of springs are provided. With reference to FIG. 2 of U.S. Pat. No. 6,029,793, the position of the various springs is crowded so that the angular displacement is limited. Moreover, the complexity of the arrangement makes it more expensive to manufacture and assemble.
Where the dampening system for dampening gear rollover noise is not sufficient for dampening the noise to an acceptable level, often the solution is to employ a dual mass flywheel. This solution will often provide sufficient dampening to overcome the gear rollover noise, but such flywheels are very expensive and are therefore not preferred.
The present invention therefore recognises the need to provide a solution to address gear rollover noise.
In respect of drive springs, prior art friction clutch assemblies typically employ straight coil compression springs to drive between the plate assembly (previously defined as the combination of the main plate and the side plate) of the clutch assembly that engages the flywheel of an engine, and the hub flange of the clutch assembly. In some other clutch assemblies, straight rubber cylinders are employed.
The preference for using straight coil compression springs arises on the basis that they are easy to manufacture and are therefore inexpensive. In addition, the clutch plate of a traditional clutch assembly has a low angular displacement (as described above) and because of this, spring forces generated in the drive springs are substantially directed along the spring axis, despite that the forces load the spring at a slight angle. Given that the spring forces are substantially loading the spring along its axis, a straight compression spring is sufficient for that purpose.
In a traditional clutch plate as described above, the drive springs can also operate without the need for guides along their length, to maintain them straight. This has the consequential benefit that the springs do not unnecessarily rub on other components of the clutch assembly, which would otherwise cause wear and generate heat, both of which can be detrimental to the life of the clutch plate.
The benefits of using straight coil compression springs in a traditional clutch plate which has low angular displacement do not apply if the clutch plate has a wider angular displacement. In that form of clutch plate, the drive springs need to be long enough to extend across the wider angle of displacement, but they also need to remain strong enough to match the torque of the engine that is to be transmitted through the clutch plate. Because the drive springs in this form of clutch plate are required to be longer than those of a clutch plate having low angular displacement, it is not ordinarily possible to fit the longer spring into the space available at the centre of the clutch plate where the shorter form of spring is usually fitted.
Moreover, if a longer straight coil compression spring is used as a drive spring in a clutch plate having a wider angle of displacement, as the spring is compressed, the spring force will no longer act along the axis of the spring and thus the normal compressive strength of the spring will not be available. The longer spring is actually forced into a trapezoidal shape by the hub flange acting against the side and main plates of the clutch plate.
Accordingly, in a clutch plate having a wider angle of displacement, the applicant has recognised the need to provide an alternative drive spring arrangement.